Broadcast in vPC

[Suprit Chinchodikar]

Hello as per the  below topology. I have two questions.
1) When S1 sends the broadcast traffic for S2, through which link the broadcast will go?
2) Also, when the broadcast reaches S2 it can send the same broadcast back to S1 as both the servers are connected with N9k1 and N9k2.
So, how do we avoid this kind of loops?

[Dawei · Accepted solution]

Hi Suprit,

1. The broadcast traffic may hash to 9k1 or 9k2, depending on the hash of S1.

1.1 If a broadcast packet hashes to 9k1, 9k1 will flood it to S2 and the peer-link. 9k2 will receive the broadcast packet but will not flood it to S2 (vPC loop avoidance mechanism: When a vPC receives a packet from the peer-link, it will not send it to vPC member ports).

1.2 If a broadcast packet hashes to 9k2, 9k2 will flood it to S2 and the peer-link. 9k1 will receive the broadcast packet but will not flood it to S2 (for the same reason).

1.3 The reason why we need to flood packets to the peer-link is that there may be some orphan ports on the peer switch, and we need to ensure that the orphan ports can receive the broadcast packet.

2. S2->S1 traffic is same as S1->S2. The traffic will be hashed to 9k1 or 9k2, and the same mechanism will be applied.

[Suprit Chinchodikar]

@Dawei,
So the hashing happens on the server level S1 right? Can we manually define which link the traffic should pass? Also  can both the switches 9k1 & 9k2 can act in active active? If yes, what can be the scenario over here? And how the broadcasting will work in this case?

[MHM Cisco World]

From view point of S1 and S2' when it receive broadcast it flood it to all port except port it receive from'

So S1 and S2 have port channel' and receive broadcast from one link of port channel' it not flood again to other link in same port-channel' it defualt behavior and can not be change.